1. Field of the Invention
The present invention relates generally to a shift control method of an automatic transmission and, more particularly, to a technique for mitigating shift shock when upshifting.
2. Description of Related Art
The ultimate goal of controlling an automatic transmission is embodying smooth shifting performance. In the conventional technique, hydraulic power transmission apparatuses, such as torque converters, have played an important role in ensuring such smooth shifting performance.
That is, such a conventional torque converter itself absorbs a large proportion of the vibrations or shocks which occurs in a power transmission system due to variations in torque when shifting gears. Thereby, smooth shifting performance can be achieved.
However, in a parallel hybrid system or the like in which an electric motor in place of a torque converter is coupled to an input shaft of a transmission by a rigid body, there is no hydraulic clutch which physically absorbs variations in torque which is induced when shifting gears. Therefore, such variation in torque just causes shift shock, thus markedly deteriorating shifting performance.
In particular, at a late stage of a shifting process, rapid variations in torque are typically induced. In the case of the conventional system provided with the torque converter, slip operation of the torque converter absorbs the majority of such rapid variations in torque, but in the case of a system having no torque converter, rapid variations in torque at the late stage of the shifting process are responsible for deterioration of the shifting performance.
For example, FIG. 1 illustrates upshifting from i gear to i+1 gear in an automatic transmission provided with the conventional torque converter. Controlling engagement side hydraulic pressure for shifting includes increasing hydraulic pressure at gradient SL1 after a fill time at an initial stage of the shifting, and rapidly increasing the hydraulic pressure at gradient SL2 greater than the gradient SL1 when it reaches a synchronization point, thus completing the shifting. At the synchronization point, a shock occurs due to differences in speed and torque which exist between an output shaft and an input shaft. Such a shock can be absorbed by the conventional torque converter. However, in the case of a system having no torque converter, such a shock is just transmitted to the power transmission system as shift shock, thus markedly deteriorating the shifting performance of the vehicle.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.